Mechanical relays have been used for years to switch high voltages from a source to a load. For example, defibrillators have relied on mechanical relays to transfer energy stored in a high voltage capacitor, via defibrillation paddles, to a patient undergoing defibrillation. While mechanical relays are the stalwart of such applications, they are not without their problems. For example, mechanical relays are, by modern standards, notoriously slow to switch. In addition, mechanical relays require high coil switching currents to achieve switching times, are subject to wearout, can be damaged or destroyed by load interruption, and can inadvertently discharge or be damaged by environmental mechanical shock (i.e. being bumped or dropped). If this weren't enough, mechanical relays are also large and very costly, and can cause sparks that could be unpleasant, to say the least, in combustible environments.
Solid state relays are known in the art, but these relays are unsuitable in high voltage applications where voltages exceeding the limit of the device (currently around 1200 volts) are required to be switched. Failure, perhaps spectacular in nature, would be the result if such a prior art relay were to be placed in a defibrillator, where voltages in excess of 5,000 volts are required to be switched.
U.S. Pat. No. 4,754,176 discloses a high voltage solid state relay that might be suitable for some applications, but has inherent problems of its own that makes this attempt wholly unsatisfactory for many other applications, including a defibrillator. For example, the solid state relay disclosed in this patent performs its switching operation so fast that it is susceptible to inadvertent triggering. While inadvertent triggering may be tolerable in some applications, it is clearly undesirable in most applications, and can be intolerable or even fatal in other applications, such as defibrillators, or elsewhere where human beings may be on the receiving end of a high voltage relay that has been inadvertently triggered.
In addition, the relay disclosed in the above patent requires a very complex and expensive transformer for operation, and does not disclose any mechanism of preventing the individual components of the relay from experiencing an overvoltage condition, due to the tolerances and other physical attributes of the components in the relay, that has the potential of destroying the component.